Date: Fri, 28 Dec 2007 13:41:54 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: marcelm@juniper.net Cc: embedded@FreeBSD.org Subject: Re: ocpbus(4) Message-ID: <20071228.134154.1136083975.imp@bsdimp.com> In-Reply-To: <595F307F-8FC5-48D5-A69C-84660A768F23@juniper.net> References: <B5E9CBD1-0F16-422C-9AFA-CC33D988630C@juniper.net> <20071228.125020.-1962668065.imp@bsdimp.com> <595F307F-8FC5-48D5-A69C-84660A768F23@juniper.net>
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In message: <595F307F-8FC5-48D5-A69C-84660A768F23@juniper.net>
Marcel Moolenaar <marcelm@juniper.net> writes:
:
: On Dec 28, 2007, at 11:50 AM, M. Warner Losh wrote:
:
: > : Existing drivers that only check DEVTYPE (which I'm sure
: > : we'll start with) will have to check DEVCLASS as well if
: > : there's going to be variation within DEVTYPE.
: >
: > Are there existing drivers right now?
:
: The e500 port uses ocpbus(4) and if we make it generic,
: I can merge it into CVS independently of the e500 code
: itself.
True.
: > FreeBSD has two models for device enumeration. One is where the
: > parent bus decides, by whatever means, and one where the device itself
: > has enough information to allow the driver to decide. I think that
: > trying to shoe-horn the second model into a situation where the first
: > model is actually better would be a disservice.
:
: When would the first be better?
When there's no information about the children in the children. For
example, pure ISA bus one must have an external enumeration. Likewise
for devices on a SPI bus, a SSC bus or an I2C bus. These "busses" are
impossible to enumerate through any method other than some kind of
table that's supplied externally. In these cases, you cannot probe
the device at all. In many cases, one can't touch the hardware at all
in a reliable way to determine if it is there or not without
potentially disrupting other devices on the bus.
Unlike PCI, in this domain these devices don't have a device ID. What
we do with them is use the hints mechanism to wire devices on the bus
to addresses on the bus (if applicable). Since this is text based on
the driver name, the actual bus has no special knowledge of the
devices that can be on the bus. The text name is passed through more
or less directly to newbus, which does the right thing. If there's a
driver of that name, then it uses it. Otherwise the device is
ignored.
With an enumeration like you suggest, one would need to create an
enumeration for each and every one of these things, compile it into
the kernel and then have some kind of translation layer that we get
for free today with newbus and bus_enumerate_hinted_bus().
Warner
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